1. Field of the Invention
The present invention relates to a method for screening the magnetic field generated by an electrical power transmission line and to an electrical power transmission line so screened.
2. Description of the Related Art
Generally, a high-power electrical transmission line is designed to operate at medium voltages (typically from 10 to 60 kV) and high voltages (typically higher than 60 kV), and currents of the order of hundreds of amperes (typically from 500 to 2000 A). The electrical power carried in these lines can reach values of the order of hundreds of MVA, typically 400 MVA. Normally, the current carried is an alternating current at low frequency, in other words generally below 400 Hz, and typically at 50-60 Hz. In general, these lines are used for transferring power from electrical power stations to urban centres, over distances of the order of tens of km (normally 10-100 km).
Typically, electrical power transmission lines are three-phase lines comprising three cables buried in a trench at a depth of 1-1.5 m. In the space immediately surrounding the cables, the magnetic field H can reach relatively high values and, at ground level (i.e. at 1-1.5 m distance from the line), a magnetic induction as high as 20-60 μT (depending on the geometrical arrangement of the cables with respect to each other) can be measured.
To avoid possible biological effects due to exposure to magnetic fields of this size generated by low-frequency (50 Hz) sources, a “safety threshold” is considered, below which the probability of biological damage can be reduced to a minimum, if not eliminated. A threshold of magnetic induction on which some national legislation is tending to become harmonized, is 0.2 μT, a value which is approximately 100 times smaller than that generated by an unshielded three phase line as indicated above.
As it is known, placing the power cables into shielding conduits can mitigate the magnetic field generated thereby.
The article by P. Argaut, J. Y. Daurelle, F. Protat, K. Savina and C. A. Wallaert, “Shielding technique to reduce magnetic fields from buried cables”, A 10.5, JICABLE 1999, considers and compares the shielding effect provided by an open-section screen, such as a sheet of ferromagnetic material, placed above the cables, and a closed-section screen, such as a conduit of rectangular section made of ferromagnetic material, placed around the cables. According to this article, attenuation factors of approximately 5-7 can be obtained with open-section screens, attenuation factors of approximately 15-20 can be obtained with closed-section screens and attenuation factors of approximately 30-50 can be obtained when the closed-section screen is formed very close to the cables (for example from a sheet of ferromagnetic material wound directly around the three cables).
Although close conduits are reported above to provide the best shielding effects, the Applicant has noticed that cable installation into close ducts is a difficult and costly operation, due in particular to the high pulling forces required, and has considered the use of two-pieces conduits, in particular conduits comprising a base and a cover, for facilitating the installation procedure. With a two-pieces conduit, after positioning the base into the trench, the cables are laid down into the base and the cover is finally leaned onto the base to substantially close the conduit. Two-pieces conduits thus allow greater laying lengths, in particular in tortuous paths, where cable pulling inside closed (single-piece) conduits would be very difficult. Moreover, two-pieces conduits allow cable inspection during and after the lay-down.
However, the Applicant has observed that a two-pieces conduit typically has an air gap at the interface between base and cover, due to unavoidable unevenness and undulations of the contact surfaces, and has verified that the presence of such an air gap may dramatically affect the magnetic shielding properties of the conduit. For example, the Applicant has verified, with a numeric simulation, that a cylindrical shielding conduit having a 1 cm-wide longitudinal air gap determines the presence, at a distance of 2.4 m, of a magnetic field that is approximately six times that measured when a completely closed conduit is used. The Applicant attributes that worsening to the break in the electric and magnetic continuity of the shield that is caused by the high electrical resistivity and high magnetic reluctance of the air gap. In other words, the shield locally looses its magnetic confining ability and is subjected to significant leakage of magnetic field lines.
The Applicant has observed that in the practice the presence of such gap cannot be avoided, not only because of the imperfect planarity of the facing surfaces of the base and cover, but also because of the presence of oxide, sand, dirt, loose material which are typically present during the installation activity in the field, which may prevent or limit the close contact between base and cover.
The Applicant has tackled the problem of improving the magnetic shielding properties of a two-pieces conduit, which is, as previously stated, a convenient solution as concern installation.